Processes for treating metal or ceramic surfaces may be divided generally into different categories. These include:                Processes that modify the physical and or chemical nature of the existing surface        Processes that remove the existing surface to generate a new surface of different chemical and or physical characteristics        Processes that generate a new surface by the deposition of materials at the existing surface.        
Processes that are employed to modify the chemical nature of the existing surface of devices include, for example, those used to nitride, carburise and carbonitride metallic devices to harden the metal surface in order to make the devices more resistant to abrasive wear. There are currently four principle methods by which titanium, titanium alloys and steels are nitrided. These are plasma nitriding (Rie et al., 1995), ion-beam nitriding (Chen and Juang, 1997), laser nitriding (Xue et al., 1997) and gas nitriding (Gil et al., 2002). The effectiveness of these methods is principally due to the facile diffusion of nitrogen into the titanium and ferrite phases in titanium and steel alloys respectively. The principle methods by which steels (and to a lesser extent titanium and titanium alloys) are carburized are plasma carburising (Dong et al., 2006), gas carburising (Li and Manory, 1995) and vacuum carburising (Chen and Liu, 2003).
Shot peening is a process whereby the physical nature of an existing device surface may be modified. In shot peening solid particulate is propelled at high velocity by means of a carrier fluid either wet or dry, typically water and air respectively, so as to impact the surface of a target substrate typically a metallic substrate. Shot peening has long since been established as a means to induce desirable stress properties in the surfaces of metallic devices wherein the impinging particles act as peening hammers causing a local plastic deformation at the surface rendering it less prone to cracking and corrosion. In addition to the significant pressures, large amounts of thermal energy, instantaneous temperatures as high as 1000° C. have been reported, are also generated locally at the surface in the vicinity of the impact.
Among those processes that modify surface chemistry by the removal of surface material such as, for example, oxides are chemical etching treatments, electro-dissolution treatments, electro-polishing treatments. Also in this category are abrasive processes such as grit blasting and sand blasting treatments. Grit and sand blasting are treatments wherein abrasive hard particles of micrometer dimensions are delivered to the surface at high velocities in fluid streams. The high kinetic energy of these particles results in high temperatures and pressures being generated locally on the device surface upon the particles impacting the surface. This also results in grains at the surface being removed resulting in atoms previously situated in the bulk now being situated at the surface. In a grit blasting process wherein the fluid stream is air and the substrate is of reactive metal, then these atoms formerly situated in the bulk will react rapidly with oxygen so as to form a new oxide layer at the surface.
Processes that deposit new materials at a surface include, for example, Chemical Vapor Deposition (CVD), electroplating, electro-polymerization, sol gel techniques and spray coating. Spray coating is a technique whereby a liquid is atomized and sprayed at a substrate. Usually the atomization process is one whereby high-pressure gas streams are used to disrupt the species to be atomized breaking it into small droplets. These drops are then carried in the gas stream to the surface. Typically the atomized species contains materials to be deposited at the surface as solutes or as suspended particles. These materials adhere to the surface as the carrier liquid evaporates usually through complex chemical coupling agents, such as silane linkages, epoxy linkages and cross-linking agents in the case of polymers, or through curing treatments that incorporate prolonged exposure to heat as for example in the case of sol-gel deposited ceramic coatings.
Shot peening and abrasive processes have been used extensively in surface science as a means to clean and condition surfaces in preparation for further treatments. A shot peening process is known for the simultaneous cleaning and painting of substrates (Kik and Schuurink, 1985). The advantage being that the delay between cleaning and painting is eliminated minimizing re-oxidation of the cleaned metal surface prior to application of the paint. Gruss and Shapiro, 1987 describe a process for the coating of printed circuit boards in which shot peening is employed to clean and condition the surface in preparation for subsequent coatings.
More recently, a number of techniques have been disclosed which use shot peening or abrasive processes as a means to modify the surface chemistry/composition of metallic and other substrates by embedding desired solid material in the surface and these techniques may be broken into three distinct methodologies.
In the first method a single type of single-phase solid particulate is used as the peening or abrasive media. In this method the shattered pieces of the particulate become embedded in the surface of the metal on impact. Such processes are mostly used to embed ceramic materials as the particles must have sufficient hardness size and mass to abrade or peen the surface. Examples include silica, alumina or calcium phosphate ceramics among others as in the patent of Arola and McCain (Arola and McCain, 2003) and that of Kuo (Kuo, 1995).
The second method also involves the use of a single type of solid particle as the peening or abrasive media but the particles themselves are comprised of multiple components usually a hard component that gives the particle mass and density and a softer component that is desired to embed in or attach to the surface on impact. Examples are to be found in (Muller and Berger, 2004; Bru-Maginez et al., 2002; Hisada and Kihira, 2004; Omori and Kieffer, 2000) and in the Rocatek™ bonding system for dental implants.
The third method is to mix different types of solid particulate media, a primary abrasive or peening material and a secondary material desired to embed or augment the surface, in the same fluid stream so that they impinge the surface simultaneously. Examples of this process may be found in (Babecki and Haehner, 1971; Chu and Staugaitis, 1985; Spears, 1988; Vose, 2006; Enbio Ltd. et al., 2008) where such processes are claimed to modify the surface composition of a variety of substrates with a number of materials including plastics, ceramics and metals. WO/2008/033867 teaches the use of grit blasting for the impregnation of metal oxide layers with solid particles delivered to the surface during a standard grit blasting treatment, the disruption caused to the surface oxide by the abrasive action allowing the smaller/softer solid particulate to become entrained in the oxide as it reforms.
These modified shot peening methodologies are limited in their surface modification capabilities for a number of reasons. Firstly the species augmenting the surface chemistry is restricted to solid materials.
In addition the augmented surface layer is a composition containing the embedded particulate and the reformed oxide of the target metal. While this presents the possibility of augmenting the surface layer of metals it is restricted to layers of approximately equal thickness to the native oxide layer on the metal substrate of interest. In many metals such as for example titanium, aluminium and alloys thereof this layer itself may be of the order of nano meters naturally limiting the concentration and nature of the desired particulate that may be incorporated into this thin surface layer.
Furthermore, the solid particulate desired to augment the surface may be in the sub-micron or nanometer size range, the handling of such solid-state particles generating respiratory and other health and safety hazards raises health and safety issues.